Linking the Abruptness of South Asian Summer Monsoon Onset to Convection Aggregation Using SP‐CAM

Abstract A key challenge in understanding South Asian summer monsoon (SASM) onset is to identify how convection–circulation coupling redistributes moist static energy (MSE) before the mature cross‐equatorial monsoon circulation is established. This study aims to explain the variability of the onset abruptness of the SASM by investigating its energetics in a 20‐year simulation of the Superparameterized Community Atmospheric Model. By conditioning on local column water vapor (CWV), we show that moisture‐dependent distributions of shallow and deep convection are essential for the emergence of a shallow circulation during the pre‐onset stage (day −25 to day −5). This shallow circulation drives up‐gradient MSE transport from dry to moist regions across a CWV threshold of approximately 35 mm. Such cross‐moisture MSE transport resembles the MSE redistribution mechanism identified in convection aggregation studies. Projecting these moisture regimes back into geographical space reveals that an aggregation‐like enhancement of spatial MSE variance occurs primarily over the western Indian Ocean. MSE variance budget analysis indicates that the increase in variance is dominated by the shallow‐circulation‐driven transport rather than diabatic processes. This transport exports MSE from the Southern Hemisphere subtropics to the Northern Hemisphere, thereby strengthening the cross‐hemispheric MSE gradient. Based on these outcomes, a set of cross‐moisture metrics is derived to quantify shallow‐circulation‐driven transport, revealing a robust relationship between transport efficiency and onset abruptness. This mechanism is further corroborated by mechanism‐denial experiments targeting the cloud radiative effect of stratocumulus over the southern Indian Ocean, demonstrating that weakened transport subsequently slows the monsoon establishment.